1. Field of the Invention
This invention relates to a control system of pumping operation using an AC exciting generator-motor, which is applied to a variable speed system of a pumped storage power plant, and, particularly, to an improved manner of stopping the pumping operation thereof.
2. Description of the Related Art
Recent pumped storage power plants have tendencies to enhance their power capacities as well as to expand their heads between upper and lower reservoirs in view of their conditions of location and/or their operational efficiencies. Conventionally, a sync machine is used as the generator-motor of the power plant, and, therefore, the rotation speed thereof is fixed at constant.
In such a power plant, when the pumping operation must stop, the wicket gate of the reversible pump-turbine is controlled to be closed or squeezed, and when the opening of the wicket gate falls under a predetermined degree, the generator-motor is disconnected (parallel CB off) from the power system.
According to a conventional pumping operation stopping control, however, the degree of squeezing the opening of the wicket gate does not proportionally correspond to the input power of the generator-motor. For instance, when the opening of the wicket gate is squeezed to 20% of the full opening, the input power is decreased to only 70-80% of the power used in the operating head or pumping head.
This means that a parallel circuit breaker (CB) must disconnect the generator-motor from the power system under a condition that the generator-motor is supplied with a certain input power. Such input power becomes large as the power capacity of the generator-motor is increased.
Under such circumstances, when the pumping operation is stopped, power fluctuations in the power system, caused by the parallel CB off of the circuit breaker, are liable to occur. Further, the parallel CB off with large input power of the generator-motor shortens the life of the circuit breaker.
An AC exciting generator-motor can be used in place of a conventional sync machine. In this case the rotor side of the generator-motor is connected to a cycloconverter, to thereby constitute a variable speed system.
FIG. 7 shows a main circuit configuration of such a variable speed system for a pumped storage power plant. In the figure, the rotor shaft of AC exciting generator-motor (IM) 2 is mechanically coupled to reversible pump-turbine 1 directly. The stator side of generator-motor 2 is electrically connected to power system 4, via parallel circuit breaker 3. Also connected to the stator side is breaking disconnecting switch 5.
Incidentally, the exciting magnetic field of AC exciting generator-motor 2 is synchronized with the frequency of system 4, but the rotation speed of the rotor thereof is independent of the system frequency.
The input side of cycloconverter 6 is electrically connected to power system 4, via circuit breaker 7 of the cycloconverter. Cycloconverter 6 converts the frequency of the power from system 4 into a prescribed frequency. The frequency-converted power from cycloconverter 6 is applied to the rotor side of AC exciting generator-motor 2.
In the above variable speed system, the degree of opening of the wicket gate of reversible pump-turbine 1 and the rotation speed thereof are controlled to be proper values in accordance with the pumping head (or simply, head) obtained at the time of pumping operation. By such control according to the wicket gate opening degree and the rotation speed, the AC exciting generator-motor can perform the pumping operation with a given amount of power corresponding to excess power in the system.
For a process from the pumping-operation state to the completely-stopped state of generator-motor 2, the key point of the above variable speed system resides in a manner of decreasing the input power of generator-motor 2.
In manner to achieve the above key point, the input power of generator-motor 2 is reduced to decrease the rotation speed of reversible pump-turbine 1 while controlling the wicket gate to be closed.
However, if the rotation speed of reversible pump-turbine 1 is largely decreased, the pump-discharge pressure of pump-turbine 1 is excessively reduced so that the operation of pump-turbine 1 enters the reverse pumping area. In the reverse pumping area, a reverse flow of water from an upper reservoir to a lower reservoir happens even if pump-turbine 1 operates in the pumping mode. When such a reverse flow happens, vibrations and/or temperature-rise due to agitating or dispersing loss in pump-turbine 1 occur. An operation with such vibrations/temperature-rise cannot be continued. Consequently, when this is the case, generator-motor 2 has to be parallel CB off (or switch-off) from power system 4, with certain input power.
Thus, for a pumped storage power plant in which numerous start/stop operations are to be done, the above-mentioned control will shorten not only the life of the pump-turbine but also lessen that of the circuit breaker (parallel CB), with substantial fluctuations in the power system connected to the pump-turbine.